A Souterrain is a type of underground construction mainly associated with the Atlantic Iron Age. Built near settlements, they were used as storage for food or as a hiding place from raiders. After being dug out, they were lined with flat stones and staircases down into their depths. Of those excavated throughout the UK and Ireland, artefacts are rare, indicating that they were merely in use temporarily. Some are very small, while others resemble passageways; my guess is that it would depend on the size required by the settlement, and how much time they had to prepare it.

I came across a souterrain along Loch Eriboll in 2012, as I was in the area doing research for my novel, The Cardinal. In these photos, you can see that, if you were walking out there at night or dusk, they could be very treacherous. My husband crawled down inside to take a picture back out; it was roomy enough for him to stand once inside (in this particular souterrain even the ceiling was lined with large stone slabs), though the narrow stone stairs and proportions, in general, indicated a much shorter population than modern humans. You can see from the photo of myself how overgrown the bracken and heather is; the entrance was nearly completely hidden; we were looking for it based on a geological map’s markings, but if we hadn’t known it was there, it would have gone completely unnoticed.

I needed a tunnel entrance in the exact location of the souterrain for the novel, so it enriched the story by fitting reality and the fantasy together perfectly!

Apparently the idea was that if you were suddenly attacked by a band of marauders with limited time before local warriors started to gather and attack them, you could go into the souterrain with a spear or sword and your valuables and anyone coming into the tunnel would be taking their life in their hands. However, I’m quite sure they had other uses, as you say.

They weren’t actually tunnels, at least those in Scotland; they were little more than stone-lined holes in the ground. Perhaps what you’re referring to is a chambered cairn / passage grave, such as Maes Howe, in Orkney. Even at that, I’ve never come across any evidence or history that would support the defensive theory. What is your source material for it, if I might ask – I would be interested to know.

Hi, Strange, I have never thought of them as anything else but tunnels, though they are undoubtedly quite low and you would only have been able to crawl or at best, go on hands and knees. The Irish ones are apparently different from the Scottish examples.
Here’s a relatively large example: http://www.tii.ie/technical-services/archaeology/publications/archaeologymonographseries/Mon-3-Ch-6-Hayes.pdf
And this gives some interesting photos of them: http://www.irishmegaliths.org.uk/seanchlocha7.htm
It says on Wikipedia:
Souterrains are underground galleries and, in their early stages, were always associated with a settlement. The galleries were dug out and then lined with stone slabs or wood before being reburied. In cases where they were cut into rock this was not always necessary. They do not appear to have been used for burial or ritual purposes and it has been suggested that they were food stores or hiding places during times of strife, although some of them would have had very obvious entrances.
Warner suggested that they were a place for non-combatants to hide during raids, perhaps to protect from arrows, slingshot, or spears. I can’t remember where I’ve heard the idea about being able to defend yourself easily at the entrance but it stands to reason that only one person can enter at a time and they will be at a disadvantage compared to the person already down in the souterrain. However, an attacker with time could easily build a fire in the entrance to smoke you out (or to death) or dig down to get you, so it would only be a sceach i mbéal bearna (a hawthorn in the mouth of a gap, a bouche-trou) at the best of times. Hope this helps! 🙂

Thanks for the article; I would guess that the size of the souterrain very much depended on the era it was built and the developments (i.e. improvements to the original design) that followed, as well as the size of the community that was building it/intending to use it.